1. Field of the Invention
The subject invention relates to optical lithography techniques and, more particularly, to optical lithography processes and structures which act to provide improved resolution in the manufacture of integrated circuits.
2. Description of the Problem and Prior Art
Optical lithography techniques typically involve the exposure of a photosensitive resist material in a pattern by light, and the subsequent formation of this pattern in the resist layer by etching. Although the fundamental limitation to spacial resolution in such a process resides in the diffraction of the radiation used which in turn depends upon the wavelength of the radiation, practical resolution is limited by the degree of scattering and reflection from the surface of the underlying substrate. The problem is compounded where a monochromatic exposure of the photoresist film over the reflecting substrate establishes a standing-wave pattern of light within the film. The standing-wave results in a significant attenuation of the intensity of the light along the thickness direction of the film, resulting in varying degrees of exposure of the photoresist. The condition becomes deleterious when using projection exposures for the production of patterns within the micron and submicron range, since excessive exposures are required to fully bring out the smaller patterns.
It is known that variation of exposure in thickness direction can be related to the square of the well-known standing-wave ratio (SWR), values for which range up to the order of 25 for bare or oxidized silicon substrates and higher for highly reflective substrates, such as aluminum films. The least favorable situation occurs for those reflective surfaces (i.e., aluminum, other metals, bare silicon, and oxidized silicon with specific oxidized thicknesses) that produce an optical node or minimum intensity at the photoresist-substrate interface, thereby requiring aggressive development or special procedures, such as post development ozone etching, to remove the photoresist residues within the micron sized images. Dimensional control of micron size images is therefore difficult to maintain, especially for densely packed image arrays.
One method of eliminating reflection from the substrate surface involves the formation of a sub-layer of highly absorbent material. Such layer acts to prevent the radiation from reaching the reflective substrate surface thereby eliminating the scattering and standing wave phenomena which act to degrade resolution. However, there are a number of requirements coupled with the use of an absorbent sub-layer. For example, the absorbing sub-layer must be compatible with the overlying resist film and underlying substrate as pertains to coating, adhesion and development. The sub-layer must also be chemically and structurally stable in the face of the overlying resist and processes for forming same, and yet thin enough so that it may readily be removed after development of the photoresist. The most logical approach is to form the sub-layer from resist material which has been loaded with a suitable dye. However, dyes typically tend to segregate out of polymer material upon drying unless their concentrations are so low as to give very small absorptions. Where dye concentrations are low, then, the thickness required must be so large as to make the removal of such layers impractical.
Another approach to reducing reflection from substrates involves the use of a double-layer photoresist film whereby only the top layer is photosensitive. In such an arrangement, a significant reduction of the SWR occurs within the top layer, and improved dimensional control occurs. Such an arrangement is described in IBM Technical Disclosure Bulletin entitled "Two and Three-Layer Photoresist Technique" by Kaplan et al, Vol. 15, No. 7, December 1972, pp. 2339-40. This article also describes the use of an interlayer between a thin-top layer and a thick-bottom layer on the substrate. The difficulty with the two and three-layer photoresist techniques described by Kaplan et al resides in the fact that they are hard to fabricate and cumbersome to use. This difficulty is, in part, due to the fact that there are interactions of the bottom layer of resist with both the solvent system of the top layer during application and with the developing solution of the top layer during development.
A further multilayer resist structure is described by W. J. Stetson in an IBM Technical Disclosure Bulletin entitled "Producing Reduced Reflectivity Master Mask on Working Plate", Vol. 18, No. 10, March 1976, pp. 3167. As is evident, the Stetson approach involves the deposition of several layers of material using reactive sputtering.
Antihalation layers which absorb light transmitted by light sensitive layers in order to prevent reflection to the sensitive layer are known in the lithographic printing art. Typical of the antihalation approaches are those described by Nadeau et al in U.S. Pat. No. 2,596,713 and Klockgether et al in U.S. Pat. No. 3,262,782. It is clear from a review of these patents that the materials involved in this art are not analogous to those involved in the fabrication of integrated circuits in micron and subsubmicron semiconductor technology. More particularly, the antihalation techniques involve the use of photographic films and plates wherein the resultant resolution in development is orders of magnitude lower than that required in the manufacture of circuits using VLSI (very large scale integration).
In accordance with the present invention, improved resolution in optical lithography, as used in VLSI, is obtained by eliminating light scattering and reflection from a silicon substrate surface into the overlying photosensitive layer by employing a chemically and structurally stable sub-layer of highly absorbent material between the photosensitive layer and substrate. More particularly, in accordance with the present invention, improved resolution in optical lithography for VLSI is obtained by employing a thin film of 4-phenylazo-1-naphthylamine as the light absorbent sub-layer. Such sub-layer is formed by the evaporation of Sudan Black B dyestuff at 10.sup.-3 to 10.sup.-4 Torr and approximately 120.degree. C. for approximately 5 minutes. Evaporation for 5 minutes and post-baking for 10 minutes at 160.degree. C. results in a thin, stable and adherent film capable of efficiently absorbing light across the entire UV spectrum, and at the same time exhibiting compatibility with the resist and substrate.
It is, therefore, an object of the present invention to provide improved resolution in optical lithography, as employed in the manufacture of integrated circuits.
It is a further object of the present invention to provide improved resolution in optical lithography such as to permit the fabrication of micron and submicron patterns on semiconductor substrates.
It is yet a further object of the present invention to provide improved resolution in optical lithography by employing an intermediate layer between the silicon substrate and photosensitive resist layer so as to eliminate substrate light reflection and therefore light scattering and standing wave patterns in the optical exposure of the photosensitive material used to form patterns on the substrate.
It is yet still a further object of the present invention to provide improved optical lithography processes and structures for the fabrication of integrated circuits by forming a light absorbing layer of 4-phenylazo-1-naphthylamine between photosensitive resist layer and semiconductor substrate for purposes of reducing reflection from the substrate when exposing the photosensitive resist material to light.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.